The present invention relates to positioning tables and, more particularly, to positioning tables of a type having a slide guided for linear travel between arms of a U-shaped frame.
In my prior U.S. Pat. No. 4,013,280, I disclose a positioning table having a U-shaped frame with a first pair of grooves aligned parallel to each other in facing arms of the U shape. A table, or slide, between the arms of the U-shaped frame includes a second pair of grooves facing corresponding grooves in the arms. Ball or roller bearings engaging the facing grooves permit displacing the slide along a linear axis defined by the grooves in the frame.
One of the arms is thick enough to resist elastic deformation while the other contains a thin section effective to provide a predetermined resilience. The bearings in the grooves are slightly oversize thus elastically deforming the thin section and producing a preload on the bearings. The preload is sufficiently large that it represents the dominant force upon the bearings.
Great accuracy and repeatability is attainable in such positioning tables due to the high precision with which contacting surfaces are formed. The grooves are ground and polished to high precision. Ball or roller bearings are of similarly high precision. The preload is adjusted to precise values by control of the ball or roller bearing diameters. That is, to increase the bearing preload, a slightly larger bearing diameter is selected.
Although the above positioning table has attained a firm position in the market place, its construction presents some difficulties. Specifically, the maximum linear travel distance is limited to about 1.5 times the length of the slide. Thus, for long travel, a correspondingly long slide is required even when the work piece to be mounted on the slide may be of modest size. Also, at each extremity of its travel, about half the length of the slide is cantilevered from the frame. A minimum cantilever of about one-quarter of the length of the slide is attained when the slide is disposed in the center of its travel range.
A longer slide has a greater mass than a shorter slide. Such greater mass is in antagonism with the desire for rapid acceleration and precise positioning. Cantilevering as much as half the slide beyond the frame reduces the support for the slide and thus permits increased flexure and vibration; both of which are undesirable in a precision positioning device.
The prior art contains examples of recirculating ball or roller bearing assemblies in which a plurality of balls or rollers fills a racetrack. Balls or rollers along a bearing surface of the racetrack support the load. At the ends of the racetrack, the balls or rollers enter or leave a return curve communicating with a return channel. If a plurality of such a recirculating bearing assembly were used on a positioning table, the slide-length problem discussed above would be substantially reduced. One possibility is positioning a recirculating bearing assembly adjacent each of the four corners of the slide.
However, conventional recirculating bearing assemblies have a negative effect on precision. As noted above, part of the high precision provided by the positioning table of the referenced patent is attainable because of the ability to control tolerances in grinding and polishing the surfaces of the facing grooves. When a plurality of recirculating bearing assemblies is substituted for the grooves on the edges of the slide, it is not possible to position the bearing surfaces of the recirculating bearing assemblies with corresponding precision. As a result, the desired uniform and repeatable preload is unattainable.